1. Technical Field
This invention generally relates to a method for fabricating semiconductors, and more particularly, a method for achieving low temperature reflow of borophosphosilicate glass ("BPSG") both during deposition and subsequent annealing.
2. Background Art
In recent years, progress in the miniaturization and multilayer interconnection of semiconductor devices has led to an increase in the aspect ratio, with the result that uneven portions on the surface of a pattern have a serious influence on the reliability of the device. For this reason, the development of a process or material for the formation of a semiconductor device which enables better planarizing of an interlayer insulation film provided between wiring layers has been a driving force behind much research.
Borophosphosilicate glass ("BPSG") films or layers exhibit good planarizing properties and are therefore especially important in highly dense dynamic random access memories ("DRAMs"), especially in advanced DRAMs having increased stacked capacitor heights and ultra high integration densities. Typically, however, BPSG reflow or annealing requires high temperature processes, which cause device breakdown because of thermal stress and unwanted dopant diffusion. Additionally, the high temperatures may be detrimental to the advanced silicides currently used for ultra large scale integrated circuits ("ULSI"), such as cobalt silicide, titanium silicide, and nickel silicide.
In order to deal with these problems, related art methods of improving the deposition temperature have included increasing the annealing time and doping the films with germanium (Ge) to reduce the processing temperature to approximately 700.degree. C. However, Ge doping requires a longer process time, therefore decreasing production, and results in lower film quality, which in turn renders it incompatible with current ULSI processing.
It has been shown in more recent studies that the reflow properties of oxide films can be enhanced by reacting silane with peroxide at a temperature in the range of 0-50.degree. C. as follows: ##EQU1## The reflow polymerized siloxane formed yields good gap filling of the interlevel metals. However, the as-deposited film is of poor quality and must be made more dense by high temperature annealing. Furthermore, the processing time is generally longer when compared to other processes. This process can only be implemented at low temperatures (0-50.degree. C.) because H.sub.2 O.sub.2 decomposes rapidly at higher temperatures. As a result, the deposition can only be implemented in multi-step processes and with lower quality films. Additionally, the process has only been developed for undoped oxides. However, boron and/or phosphorus doped oxides are required in many applications for mobile ion gettering and to allow high etch rates during contact hole etching. Incorporation of higher concentrations (&gt;5 wt %) of boron and phosphorus dopants may increase crystal defect formation on the deposited film. This will subsequently have a detrimental effect on ULSI and VLSI device fabrication.